Exhaust gas discharged from an internal combustion engine of, for example, an automobile contains toxic components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Hitherto, three-way catalysts have been used for removing such toxic components for detoxifying the exhaust gas.
Such three-way catalysts include a noble metal (e.g., Pt, Pd, or Rh) serving as a catalytically active component; a material such as alumina, ceria, zirconia, or oxygen-occluding ceria-zirconia composite oxide, serving as a carrier; and a catalyst support made of a ceramic or metallic material and having a shape of honeycomb, plate, pellet, etc.
Recently, the regulation of automobile exhaust gas has become more strict, and the demand and prices of Pt and Rh, which are noble metals serving as a main catalytically active component of internal combustion engine exhaust gas purification catalysts, have risen. Since a rise in Rh price is a critical issue, efforts have been made on reduction of exhaust gas purification catalyst production cost by use of inexpensive Pd as a catalytically active component, and various means therefor have been proposed (see, for example, Patent Documents 1, 2, and 3). Meanwhile, there are some cases where a catalyst carrier made of aluminum borate is used. In one case of such a catalyst, a catalyst component is deposited on a powder compact which is covered with aluminum borate whiskers and which includes voids therein, whereby gas diffusivity is enhanced (see Patent Document 4).
However, it has been reported that Pd causes impairment in exhaust gas purification performance by sintering thereof at high temperature in a reducing atmosphere. Thus, suppression of Pd sintering is an inevitable issue for designing coming catalysts of a noble-metal saving format. Also, aluminum borate whiskers, having an acicular shape, have a small specific surface area, which causes cohesion of noble metal elements after long-term use of the relevant catalyst under high-temperature conditions. That is, durability of the catalyst is unsatisfactory.
In order to solve such problems, the present applicant previously developed an exhaust gas purification catalyst which exhibits excellent NOx purification performance, particularly in a rich region. The catalyst includes a carrier containing a substituted aluminum borate in which 2.5 to 11.5 at. % aluminum atoms are substituted by Fe, Co, Ga, or Ni atoms, and Pd supported on the carrier (see Patent Document 5).